Running test on existing database?

Hi,
need some input on testing database-related code. Every night we mirror the production database to a local database server. I know that this may not be 100% kosher, but we would like to run CI4 tests using this database and not having to go through the hassle of setting up migrations and seeds for a pure testing database. Our CI/CO systems also have connectivity to the database mirror.
Setting up the testing database as described on https://codeigniter.com/user_guide/testi...abase.html nukes the database after one run, not good.
We have tried defining migrations and seeds, but with 100+ tables and relations, this is getting complex very quickly. We have also tried adding mysql restore commands but the database is 18 GB in size and it gets pretty slow.
Any input Greatly Appreciated!
/Mattias